In recent years, in order to confront global environmental issues caused by, for example, exhaust gas, fuel efficiency has been improved by reducing the weight of vehicle bodies of transporters, such as automobiles. Furthermore, in order to increase safety in the event of a vehicle collision without inhibiting this weight reduction of vehicle bodies as much as possible, a technology for partially replacing a conventionally-used material with another material has been studied. Examples that have been carried out include partially replacing a steel material, which has been conventionally used, with a light alloy material that is lighter in weight and has high energy absorbency, such as aluminum alloy material or a magnesium material, or using high-tensile steel with higher strength in a part of the frame.
Normally, it is difficult to weld different types of metallic materials to each other. For example, when welding aluminum and steel to each other, Fe2Al5, which is a high-hardness and extremely-fragile intermetallic compound layer of Fe and Al, is produced at the joint interface, thus causing the joining strength to decrease.
Therefore, there has been studied a joining structure obtained by inserting a steel connection member into an aluminum frame and welding the steel connection member to another frame composed of steel (Patent Literature 1). In another disclosed joining structure, a first member and a second member are joined to each other by spot-welding the end of a rivet clinched to the first member to the second member, so that dissimilar metallic materials are joined to each other while suppressing the occurrence of intermetallic compounds (Patent Literature 2).
Furthermore, there has also been studied disposing resin at the inner side of a part where metallic members are combined with each other so as to increase rigidity and shock absorption (See Patent Literatures 3 and 4).